The present invention relates to a method for producing a particle film. More particularly, the present invention relates to a method for continuously producing a particle film and crystallized particle film comprising particles arranged in order in terms of crystallization which are useful in the areas of highly functional catalysts, sensors and transducers, various optical materials such as interference film, reflective film, reflection preventive film, 2-dimensional particle multi-lens, light adjusting film, color developing film, various electronic materials such as conductive film, electromagnetic shielding film, LSI (Large Scale Integration) board, semiconductor laser solid element and optical and magnetic recording medium, photographic material such as highly sensitive photographic paper, selective transmission film, molecular sieve and selective adsorption film.
Thin film technologies for producing a single-, or a multi-layered particle film as one form of assembly at a high accuracy and efficiency wherein particles exert their intrinsic useful functions to the greatest extent possible have been conventionally used in the areas of highly functional catalysts, sensors and transducers, various optical materials such as interference film, reflective film, reflection preventive film, 2-dimensional particle multi-lens, light adjusting film, color developing film, various electronic materials such as conductive film, electromagnetic shielding film, LSI board, semiconductor laser solid element and optical and magnetic recording medium, photographic materials such as highly sensitive photographic paper, selective transmission film, molecular sieve and selective adsorption film. Further, new thin film technologies capable of giving new physical properties and functions not found in individual particles per se to two-dimensionally assembled particles are actively introduced in the above-said industrial areas.
A number of particle film production methods are currently studied, and a suitable one is selected according to the production environment. They include the solution system such as electrolytic precipitation, interface system such as LB (Langmuir-Blodgett) film, vacuum system such as deposition and CVD, and dispersion system such as coating and spin coat.
Of these methods, the dispersion methods such as producing particle film from a particle dispersion system such as emulsion and suspension by drying and solidification include the above-mentioned spin coat, coating, and dipping techniques. These are generally used as a practical method.
Actually, however, it is difficult for the dispersive thin film systems such as the above spin coat, coating and dipping techniques to control thickness, number of layers, and particle density of particle film at a high accuracy and simultaneously in the production of thin film.
For example, the spin coat method allows production of very thin particle film but it is very difficult to control particle density. The coating method realizes a high particle density but produces only very thick film.
This means that the conventional thin film production methods such as spin coat, coating, and dipping methods are unable to produce thin film comprising the marginal thickness of a single layer of particles and high quality and highly controlled thin film such as dense and uniform particle film and crystallized particle film. Further, it is impossible for the above conventional methods to produce a large amount of thin film continuously.
In view of these circumstances, the inventors of the present invention have previously proposed a thin film forming method to solve the above problems of the thin film production method of the dispersive thin film system.
This is a method to produce particle film and crystallized particle film by evaporating wetting film and is a method to form 2-dimensionally assembled, uniform and dense particle film.
In the above method to produce particle film by evaporating wetting film, particle film is formed in the manner described below, for example. In FIG. 17(a), fine particles (1) of 2R in diameter are immersed in a liquid film (2) whose thickness is h (2R less than h) on a flat board (3). This liquid film (2) is then thinned to a thickness of 2R greater than h, as shown in FIG. 17(b). Two-dimensional self-assembly of fine particles (1) starts to form thin film of particles at this moment.
Two factors are working in the process of this two-dimensional assembly: lateral capillary force deriving from surface tension and the force generated by the flow of liquids as a result of evaporation of liquids at the wetting film. When these two forces are balanced, fine particles will be two-dimensionally assembled regularly and very quickly.
The inventors of the present invention have proposed some devices to produce stable wetting film. For example, in FIG. 18, the liquids in liquid film (2) containing particles (1) are evaporated to form thin wetting film on a flat board (3). Further, in FIG. 19, the liquids in the liquid film (2) containing particles (1) placed on a flat substrate (3) are removed by suction to form thin wetting film on said flat board (3). In FIG. 20, liquids containing particles (1) are dropped onto a substrate (3) comprising mercury, and thin wetting film is formed via wet spreading.
Although these devices have contributed very much to the basic analysis of two-dimensional assembly of particles taking place in wetting film, it is impossible to produce stable wetting film of a large area qualifying for industrial applications with the above devices. Further, it is difficult for these devices to continuously produce a large quantity of particle film because a practical method and means has not been established to supply particles to keep the process going.
Accordingly, a method to produce stable wetting film of a large area, control of the number of particle film layers, and a method to supply fine particles must be established to apply the particle film production method to an industrial scale, assisting in the production of a large quantity of particle film continuously.
The present invention was developed in consideration of the above circumstances and solves the problems in the conventional particle film production methods by providing a method for producing a large quantity of particle film continuously. Said method is characterized by the ability to produce stable wetting film of a large area, control the number of particle film layers and supply fine particles efficiently and accurately, allowing the new particle film production method through self-assembly of fine particles to be applied on an industrial scale.
With a view to solving these conventional problems, the present invention provides a novel method for producing a particle film by contacting a solid or liquid substrate with a particle dispersive suspension, and sweeping, spreading and moving the leading edge of a meniscus formed at the 3-phase contact line by atmospheric air or gas, substrate and suspension, thereby forming the particle film, wherein the particle density and the number of particle film layers are controlled by the traveling velocity of the leading edge of the meniscus, volume ratio of particles and liquid evaporation rate, using these as parameters.
More specifically, with the present invention, particle suspension is spread on a solid or liquid substrate, stable wetting film is formed near the 3-phase contact line at the leading edge of the meniscus formed by the substrate, suspension and air, and the particles are closely packed in said wetting film by the assembling force of the particles generated by the flow of the liquids and the lateral capillary force, in which process the 3-phase contact line is continuously swept under controlled conditions to continuously produce particle film in one direction.